questions fm antenna matching transformer and their connectors.

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scrapjack+
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questions fm antenna matching transformer and their connectors.

Post by scrapjack+ » 12 May 2019 05:33

I have seen the twin fork to coaxial adapters that say they are for FM reception. Some say they turn 75 ohm into 300 ohm. I read that has something to do with impedance matching however I have been unable to find a site that actually explains what these adapters are for aside from converting two types of connectors, old TV's had both. Can some one provide me with a basic explanation of what purpose these actually serve? I know that FM signals were so close in frequency range to TV broadcast signals that FM traps were made to filter it out so FM wouldn't appear as static on TVs. This is why a old set of rabbit ears, or other TV antennas, do wonders for FM radio reception now that TV broadcasts have stopped.
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I want to add a panel mount connectors to a radio that was formally a cabinet record player with no external antenna input. The original antenna was a large "twin antenna wire" loop along the center record storage space of the cabinet, but that wiring was destroyed years ago, so there was no salvaging it or getting electrical measurements. I can't seem to find those panel mount fork terminal connectors unless they are pulled from turntables listed at absurd prices. Panel mount coax connectors I have plenty of from dead electronics. If I can use coax it simplifies making the back panel, and installing/removing antennas whenever it has to be moved. I can always use banana jacks instead of forks for the turntable ground wire. To find those fork antenna connectors I've tried searching for panel mount screw block terminals, is there another name that I should be searching for?

I know with my CB radio I had to tune my antenna to the radio with a SWR meter or it would burn its receiver out when I transmitted. Can a FM radio that does not transmit be damaged by an (greatly) incorrectly matched antenna? I think I have a low power car FM transmitter for mp3 players. Should I broadcast a sine wave and connect a set of rabbit ears to the radio interrupted by my SWR meter? Will that yield some information that would be useful to setting up the internal wiring for the FM radio's antenna? FM reception where I live is severely afflicted by the surrounding mountain ranges.

Thank you for your time, and any help provided.

Coffee Phil
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Re: questions fm antenna matching transformer and their connectors.

Post by Coffee Phil » 13 May 2019 04:17

Hi scrapjack,

Those little devices are 2:1 autotransformers used to convert from 75 Ohm unballanced to 300 Ohm ballanced. The impedance ratio is the square of the turns (or voltage) ratio, hence a turns ratio of two is an impedance ratio of four.

Phil

scrapjack+ wrote:
12 May 2019 05:33
I have seen the twin fork to coaxial adapters that say they are for FM reception. Some say they turn 75 ohm into 300 ohm. I read that has something to do with impedance matching however I have been unable to find a site that actually explains what these adapters are for aside from converting two types of connectors, old TV's had both. Can some one provide me with a basic explanation of what purpose these actually serve? I know that FM signals were so close in frequency range to TV broadcast signals that FM traps were made to filter it out so FM wouldn't appear as static on TVs. This is why a old set of rabbit ears, or other TV antennas, do wonders for FM radio reception now that TV broadcasts have stopped.
antenna 2.jpg

I want to add a panel mount connectors to a radio that was formally a cabinet record player with no external antenna input. The original antenna was a large "twin antenna wire" loop along the center record storage space of the cabinet, but that wiring was destroyed years ago, so there was no salvaging it or getting electrical measurements. I can't seem to find those panel mount fork terminal connectors unless they are pulled from turntables listed at absurd prices. Panel mount coax connectors I have plenty of from dead electronics. If I can use coax it simplifies making the back panel, and installing/removing antennas whenever it has to be moved. I can always use banana jacks instead of forks for the turntable ground wire. To find those fork antenna connectors I've tried searching for panel mount screw block terminals, is there another name that I should be searching for?

I know with my CB radio I had to tune my antenna to the radio with a SWR meter or it would burn its receiver out when I transmitted. Can a FM radio that does not transmit be damaged by an (greatly) incorrectly matched antenna? I think I have a low power car FM transmitter for mp3 players. Should I broadcast a sine wave and connect a set of rabbit ears to the radio interrupted by my SWR meter? Will that yield some information that would be useful to setting up the internal wiring for the FM radio's antenna? FM reception where I live is severely afflicted by the surrounding mountain ranges.

Thank you for your time, and any help provided.

lenjack
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Re: questions fm antenna matching transformer and their connectors.

Post by lenjack » 13 May 2019 18:11

The FM band is located between channels 6 and 7 of the old vhf TV broadcast band - channels 2-13. Some old rooftop TV antennas would receive the FM band, but some would block it out. Hope this is helpful.

zlartibartfast
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Re: questions fm antenna matching transformer and their connectors.

Post by zlartibartfast » 13 May 2019 18:42

I have never heard of an FM receiver being damaged by it's antenna, though I suppose if someone were to hook up a gain amp made for TV then crank it all the way, something bad could happen.

If you have problems with signal reflection (multipath interference) then use a directional antenna rather than a dipole.
Yes, those old TV antennas (the ones with the longer spikes, made for analog TV) do a fair job with FM reception - main thing it to have it as high above the ground as you can reasonably get and be able to rotate it.

this website may be of some help:
https://antennaweb.org/
yes, it's for TV broadcasts, but in many cases the FM towers are in the same vicinity as the TV towers. your mileage may vary....

scrapjack+
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Re: questions fm antenna matching transformer and their connectors.

Post by scrapjack+ » 14 May 2019 21:01

Coffee Phil wrote:
13 May 2019 04:17
Hi scrapjack,

Those little devices are 2:1 autotransformers used to convert from 75 Ohm unballanced to 300 Ohm ballanced. The impedance ratio is the square of the turns (or voltage) ratio, hence a turns ratio of two is an impedance ratio of four.

Phil
Searching for, "balanced vs unbalanced antennas" seems to yield allot more useful results. Such as, http://www.antenna-theory.com/definitions/balun.php which states that dipole antennas must be balanced to function properly.

Apparently the wiring that used to be an antenna in the case was called, "300 ohm twin lead," which has a nice article in wikipedia. I suppose that means the antenna should be a 300 ohm balanced antenna. The antenna connector was just two male spade connectors. According to wiki using the wrong impendence via the coax adapter yields only slight diminishment in reception, it is in transmission that it yields problems. Since I live in an area of bad reception, I will accept the inconvenience of forks.

Thank you everyone for the help with the antenna pairing. I think I have enough information on that now.

I still would like some help on the connector question. This is the second thread I have asked, and I still haven't gotten an answer. I don't want to start a whole tread asking just this one question. Is there a name that merchants use for the screw type connectors used for panel mounting forks, like what is on the coax adapter, or the ones in this photo? It is the same type of connector used for the phono ground wires. This seems like it should be the easy question. I'm about ready to make my own with some copper plates and a machine screw wood anchor if I can't find them at a reasonable price. By what names will the sellers list these connectors? I do not mean to be rude, and hope My words are not interpreted that way. It just seems like people ignore this question for the more interesting ones, and this time I have a time concern for getting an answer.
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lenjack
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Re: questions fm antenna matching transformer and their connectors.

Post by lenjack » 14 May 2019 21:05

I think you've got it right, but just in case, the 300 BAL is your twin lead.

Jim Leach
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Re: questions fm antenna matching transformer and their connectors.

Post by Jim Leach » 15 May 2019 00:37

Home Depot sells screw terminal blocks. Not exactly what you want (try searching “binding posts”) but would certainly work.

They come with 4-6 or 8 terminals. They are connected across (so top to bottom if set horizontally) so you can connect to the “top” and go internal and use the “bottom” for external connections.

scrapjack+
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Re: questions fm antenna matching transformer and their connectors.

Post by scrapjack+ » 15 May 2019 00:57

Jim Leach wrote:
15 May 2019 00:37
Home Depot sells screw terminal blocks. Not exactly what you want (try searching “binding posts”) but would certainly work.

They come with 4-6 or 8 terminals. They are connected across (so top to bottom if set horizontally) so you can connect to the “top” and go internal and use the “bottom” for external connections.
I didn't realize the screw block terminals were connected so. If I need 3 separate connections I'd need two blocks I suppose, not ideal. I had thought "binding posts" only referred to female banana jacks. I see a few listings now that say they handle both. Thank you.

Jim Leach
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Re: questions fm antenna matching transformer and their connectors.

Post by Jim Leach » 15 May 2019 01:39

No, if you need three connections, you can get a four terminal block (and have an extra). If you want to connect adjacent terminals, you add jumpers.

I’ll see if I can find it. Once you see it, you’ll understand better. Hard to explain.

Jim Leach
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Re: questions fm antenna matching transformer and their connectors.

Post by Jim Leach » 15 May 2019 01:40

Here you go:

https://www.newark.com/molex/38770-0108 ... 5659266121

This is what I’m talking about.

scrapjack+
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Re: questions fm antenna matching transformer and their connectors.

Post by scrapjack+ » 15 May 2019 02:56

Ok, I see. I thought you meant horizontally, not vertically.

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Re: questions fm antenna matching transformer and their connectors.

Post by scrapjack+ » 16 May 2019 05:02

A quick warning. I bought some of the banana jack binding posts that say they work for forks, the ones made to fit gang boxes. I found out they require the fork to go through the center hole resulting in an off center mounted wire, for standard rabbit ear forks at least. This probably won't bother most people, but for some it will. Also if it is one with the corner connectors, there isn't enough room on either side of the plastic molding to fit a socket, if you wanted to rearrange them.
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Project is moving along, slowly.

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